Dunno if this works for anyone else, but I just played through the castle level attempting to get the "Defend the Doctor" achieve. I didn't even think to look for the FSB agents, so I never saved them from their cells. Unfortunately, during the final firefight where you need to secure the landing zone, the doc decided to go lemming on me and repeatedly ran out into the open no matter how many times I reloaded the checkpoint. Anyway, once I finished the level the "Prisoner Release Programme" achievement unlocked. I guess technically all of the FSB agents did survive, I just never rescued them...

Wow...that's a good tip for anyone who have a trouble to save and protect them in order to get the achievement but you gave an easy way out. I think you should put it in the achievement guide to request to add it.